Method and composition of a hydrophilic polymer dispersion containing a colloidal silica to be used for the retention aids of fine particles in papermaking system

ABSTRACT

The present invention relates to the method and composition of a hydrophilic polymer dispersion containing a colloidal silica to be used for the retention aids of fine particles in papermaking system. More particularly, it relates to the method and composition of a hydrophilic polymer dispersion containing a colloidal silica, which can be used for recycling resources and reducing pollution of the waste water by retaining fine particles in papermaking system.

TECHNICAL FIELD

The present invention relates to the method and composition of ahydrophilic polymer dispersion containing a colloidal silica to be usedfor the retention aids of fine particles in papermaking system. Moreparticularly, it relates to the method and composition of a hydrophilicpolymer dispersion containing a colloidal silica, which can be used forrecycling resources and reducing pollution of the waste water byretaining fine particles in papermaking system.

BACKGROUND ART

In the paper industry, the energy expense reaches 10˜15% of cost. Thelarge quantity of waste water occurs due to the use of much water inproduction. Especially, the accumulation of anionic trash in the systeminduces the declination of retention of fine particles. Therefore, incase that raw material including a lot of fillers contaminates the wastewater, the cost for treatment may increase.

Not a few retention aids have been developed, but they have to beapplied to the paper having high quality, and they occur much wastewater. Further, more than two materials of retention aids have to beused, because more than two points have to be input respectively. Hence,it has a lot of drawbacks of: i ) difficulty of controlling process, ii)low stability of operation, and iii) production of the paper having lowquality.

For example, there are many kinds of retention aids based upon variousmaterials, such as, polyacrylamide, polyamine, polyamidoamine,polyethyleneimine and polyethyleneoxide. Among them, the polyacrylamidederivative used for retention aid or paper strength agent can beprepared by the polymerization reaction of dimethylamine, formaldehydeand homopolymer of acrylamide.

The efficiency of polyacrylamide is improved in addition to theparticle, such as, bentonite and colloidal silica. Further, in thecationic polymer dispersion, the content of cationic monomer ispreferably 10˜20 mole % due to the low cationicity. Therefore, the flockhaving large size and low density is obtained by bridging flocculation,and the paper having low quality is obtained. The excess input ofretention aids induces the reduction of fiber formation on the paper. Incase that the anionic trash is accumulated in the system, theflocculation is reduced.

Allied Colloids Co., Ltd. provides the ‘Hydrocol’ system as a retainingmethod, which has to inject each cationic polymer and each bentoniterespectively. Therefore, it has a lot of drawbacks of: i) limited rangeof application, ii) limited effect to specific paper due to its highcationicity, and iii) large selectivity of effect. Further, it can beused by following steps of: i) forming the flocculation by addingcationic polymer to pulp slurry, ii) destroying the flocculation byapplying strong shear force, and ii) inducing the reflocculation byadding bentonite to said flocculation.

Akzo Nobel Co., Ltd. provides the ‘Compozil’ system using cationicstarch and colloidal silica, and the ‘Compozil-P’ system using cationicpolymer and colloidal silica. In case that the cationic starch in thisproduct is flowed out of the system of ‘Compozil’, it has a lot ofdrawbacks of: i) increasing the pollution load of the waste water, andii) increasing the cost for the retention. In the system of‘Compozil-P’, a diameter of said colloidal silica has to be less than 10nm, and the solid content of said colloidal silica has to be less than10%. Hence, a large amount of transport cost is required. Further, thecolloidal silica structured by linear branch of globular silica isunstable, and colloidal silica having less than 3% of solid content hasto be provided.

Meanwhile, all developed retention aids can be classified into thefollowing groups of: i) retention aid comprisingpoly-diallyldimethylammoniumchloride or copolymer of acrylamide anddiallyldimethylammoniumchloride; ii) microparticle system comprisingorganic coagulant and cationic polyacrylamide; iii) microparticle systemcomprising organic coagulant and anionic polyacrylamide; iv) retentionsystem comprising starch and cationic polymer; and v) retention aidcomprising solubilized polymer. However, it has a lot of problems of: i)maintaining the retention of raw material, and ii) inducing thepollution. Further, the retention aid influences the quality of paper,for example, strength and degree of whiteness.

The aqueous dispersions of water-soluble nonionic and anionicallycharged vinyl and allyl addition polymers were disclosed in WO 97/34933.Also, they can be obtained by polymerizing in the presence of ananionically charged water-soluble polymer stabilizer in a saturated saltsolution.

Further, the polymerization process of preparing a water-soluble polymerin the form of a water-continuous dispersion was described in EuropeanPat. 0 630,909 A1. In this process, the peak in-production viscositydeveloped in the aqueous reaction mixture is lower than the peakin-production viscosity developed in an equivalent batch polymerization.

For forming dialkylaminoalkyl (meth)acrylamide polymer dispersions, anovel dispersant system was disclosed in U.S. Pat. No. 5,597,859.Further, European Pat. 0 839,767 A2 suggested a method for clarifyingink-laden water obtained from the recycling of paper stocks by treatingsaid water with a conventional coagulant followed by treatment with ahydrophilic dispersion polymer. In this method, the hydrophilicdispersion flocculant is a copolymer of DMAEA.MCQ [dimethylaminoethyl(meth)acrylate methyl chloride quaternary] and (meth)acrylamide.

The improved process for the preparation of water soluble polymerdispersion was disclosed in European Pat. 0 657,478 A2.

However, none of prior references discloses the far enhanced retentionproperty of fine particles compared to formerly developed hydrophilicpolymer dispersion

DISCLOSURE OF INVENTION

The present invention relates to a hydrophilic polymer dispersioncomposition containing a colloidal silica and a method for preparingsuch composition which comprises the following steps of: i) mixingacrylamide, cationic monomer, anionic monomer, polymer of cationicmonomer, anionic salt and colloidal silica, ii) 1st-polymerizing themixture obtained in previous step, iii) 2nd-polymerizing the1st-polymerized mixture containing unreacted monomers completely, andiv) adding and mixing anionic salt to obtained polymers.

Further, the hydrophilic polymer dispersion containing a colloidalsilica of the present invention is prepared by following steps of:

i) mixing 1.0˜25 wt % of acrylamide, 0˜2 wt % of anionic monomerselected from the group consisting of acrylic acid, methacrylic acid anditaconic acid, 1.0˜30 wt % of cationic monomer mixture of compound offormula I and formula II, 0.5˜5 wt % of polymer selected from the groupconsisting of homopolymer of compound of formula I, homopolymer ofcompound of formula II and copolymer of compound of formula I andformula II, 10˜30 wt % of anionic salt, 0.5˜10 wt % of colloidal silica,0.01˜1 wt % of nonionic surfactant, 0.05˜2 wt % of dispersion stabilizerand 40˜75 wt % of water

ii) 1st-polymerizing the mixture in addition to 0.001˜0.1 wt % ofpolymerization initiator to said mixture;

iii) 2nd-polymerizing the 1st-polymerized mixture containing unreactedmonomers completely; and

iv) adding and mixing 10˜30 wt % of anionic salt to obtained polymers.

 wherein

R₁ is hydrogen atom or methyl;

R₂ and R₃ are each independently alkyl group having 1 to 3 carbon atoms;

A₁ is oxygen atom or NH;

B₁ is alkylene group having 2 to 4 carbon atoms or hydropropylene; and

X₁ is anionic counter ion.

 wherein

R₄ is hydrogen atom or methyl;

R₅ and R₆ are each independently alkyl group having 1 to 2 carbon atoms;

R₇ is hydrogen atom or alkyl group having 1 to 2 carbon atoms;

A₂ is oxygen atom or NH;

B₂ is alkylene group having 2 to 4 carbon atoms or hydropropylene; and

X₂ is anionic counter ion.

BEST MODE FOR CARRYING OUT THE INVENTION

The conventional retention aid contains anionic polymer, acrylamide ornonionic polymer, such as, starch, whereas the retention aid of thepresent invention further contains colloidal silica. Further, thepresent invention enhances the degree of polymerization of the polymerdispersion by adding and mixing nonionic surfactant to the mixture ofcation monomers.

Acrylamide or methacrylamide can be used as nonionic monomer, andacrylic acid, methacrylic acid or itaconic acid can be used as anionicmonomer.

The cationic monomer of formula I and cationic monomer of formula II canbe mixed in a ratio of 10:0 to 2:8.

The reacting materials for the polymer are a cationic monomer of formulaI, cationic monomer of formula II, and homopolymer of cationic monomerof formula I, homopolymer of cationic monomer of formula II, orcopolymer of cationic monomer of formula I and formula II as polymer inthe present invention.

Said monomer is soluble in initial salt solution, but polymer materialis insoluble in initial salt solution. Therefore, the anionic salt hasthe function for preparing dispersion by dispersing polymer materials tosmall particles. Said anionic salt is selected from the group consistingof ammonium sulfate, ammonium chloride, sodium sulfate, magnesiumsulfate, aluminium sulfate, ammonium hydrogenphosphate, sodiumhydrogenphosphate, and potassium hydrogenphosphate. Further, half ormore of said anionic salt can be added before polymerization reaction,and the remaining said anionic salt has to be added after polymerizationreaction. It is proved as the desirable preparation method for thestable polymer dispersion experimentally.

The colloidal silica firstly introduced in the present invention has thefunctions of improving the retention. The commercially marketedcolloidal silica is classified into cationic colloidal silica, anioniccolloidal silica and nonionic colloidal silica. Especially, thecolloidal silica having 10˜30 nm of diameter can be used in the presentinvention. It is proved as the desirable diameter for the stable polymerdispersion experimentally.

Said polymer material and colloidal silica can be stabilized bystabilizer, such as, nonionic surfactant and glycerin. The particles ofdispersed polymer material is stabilized by following mechanisms of: i)steric hindrance of said nonionic surfactant and ii) repulsion caused byhigh charge of the polymer material. In case that the content ofnonionic surfactant is less than 0.01 wt %, stability of dispersionobtained is lowered. Further, in case that the content of nonionicsurfactant is more than 1 wt %, wasted surfactant remains. Also, 0˜2 wt% of glycerin can be used as subsidiary dispersant. In case that thecontent of glycerin is more than 2 wt %, glycerin is wasted. Further,the nonionic surfactant of the present invention activates the nucleiformation, which results in the reduction of in-production viscosity andthe stabilization of polymer particles caused by the steric hindrance ofthe surfactant. Therefore, the nonionic surfactant has a role to enhancethe mobility of the final product. Further, the chelating agent, suchas, ethylenediaminetetraacetate can be added for the formation ofcomplex. The polymer dispersion is stabilized by forming complex.

The azobis initiator, which is a kind of radical initiator, can be usedfor polymerizing said cationic monomer of formula I and formula II andsaid anionic monomer. Azobis isonitrile, for example, commerciallymarketed V-50 can be used as azobis initiator. 0.001˜0.1 wt % of saidpolymerization initiator can be used. In case that the content ofinitiator is less than 0.001 wt %, there are some drawbacks of: i) notmaintaining polymerization reaction and ii) obtaining insoluble polymermaterial having high degree of polymerization. On the other hand, incase that the content of initiator is more than 0.1 wt %, there are somedrawbacks of: i) increasing and generating the heat during thepolymerization, ii) decreasing the yield of polymer material, and iii)preparing polymer material having low degree of polymerization

Further, the hydrophilic polymer dispersion containing a colloidalsilica of the present invention is prepared by following steps of:

[mixing step] i) mixing 1.0˜25 wt % of acrylamide, 0˜2 wt % of anionic.monomer, 1.0˜30 wt % of cationic monomer mixture of compound of formulaI and formula II, 0.5˜5 wt % of polymer selected from the groupconsisting of homopolymer of compound of formula I, homopolymer ofcompound of formula II and copolymer of compound of formula I andformula II, 10˜30 wt % of anionic salt, 0.5˜10 wt % of colloidal silica,0.01˜1 wt % of nonionic surfactant, 0.05˜2 wt % of dispersion stabilizerand 40˜75 wt % of water in the reactor, ii) nitrogen purging in thereactor, and agitating said mixture more than 30 rpm;

[initiator adding step] i) heating said mixture at 20˜50° C., ii) adding0.001˜0.1 wt % of initiator to said mixture;

[1st-polymerizing step] 1st-polymerizing said mixture at 20˜50° C. for3˜6 hours;

[2nd-polymerizing step] 2nd-polymerizing the 1st-polymerized mixturecontaining unreacted monomers completely by adding 0.001˜0.1 wt % ofinitiator to said mixture for 3˜15 hours;

[finishing step] i) adding the remaining anionic salt to the polymer,ii) adjusting pH of the polymer less than 4.0, iii) sieving the polymerusing 40˜100 mesh.

In the mixing step, the inside of a reactor is purged with nitrogen, andthe raw materials are mixed and agitated homogeneously. Particularly, apart of anionic salt is added in this step, and the remaining part ofanionic salt is added after the polymerization. This mixing step isimportant for raising stability of the dispersion.

In the initiator adding step, the initiator, for example, azobisinitiator, such as, azobis isonitrile or redox initiator, such as,ammonium persulfate and sodium bisulfite is added for polymerizinganionic monomer, cationic monomer of formula I and II. Said mixture hasto be heated at 20˜50° C. where the polymerization is initiated.Further, in the 1st-polymerizing step, the reactor has to be cooled toachieve the maintenance of temperature at 20˜50° C. This step has to bemaintained for 3˜6 hours until polymerization is finished.

In the 2nd-polymerizing step, unreacted monomers in 1st-polymerizingstep is completely reacted to form the polymer in addition to saidinitiator at 20˜50° C. for 3˜15 hours. Therefore, the polymer dispersionis obtained.

The final product is obtained by the finishing step. In case that the pHof polymer dispersion is more than 4.0, the stability of dispersion islowered and the efficiency of dispersion is declined.

The final product of the present invention shows the enhanced retentionproperty compared to the commercially marketed products, for example,‘Hydrocol System’ by Allied Colloids Co. and ‘Compozil System’ by AkzoNobel Co.,. Especially, the properties, e.g. whiteness, surface strengthof paper have been excellently increased.

The present invention can be more specifically explained by thefollowing examples. However, it should be understood that the examplesare intended to illustrate, but not to limit the scope of the presentinvention in any manner.

EXAMPLE 1 Preparation of the Polymer Dispersion

The polymer dispersion is prepared by following steps of: i) mixing211.015 g of 50% acrylamide aqueous solution, 55.616 g ofdimethylaminoethyl acrylate benzyl chloride quaternary monomer (80%),7.5 g of dimethylaminoethyl acrylate methyl chloride quaternary, monomer(20%), 1.875 g of dimethyldiallyl ammonium chloride polymer (40%),164.943 g of ammonium sulfate, 0.75 g of aromatic ethoxylate, 4.5 g ofglycerin, 2.5 g of anionic colloidal silica (30%) and 464.244 g of waterin the 5-neck flask, ii) nitrogen purging in the reactor, and agitatingsaid mixture to 200 rpm, iii) heating said mixture at 35° C., iv) adding0.03 g of azobis isonitrile initiator (VA-044) to said mixture, v)1st-polymerizing said mixture at 35° C. for 3˜6 hours, vi)2nd-polymerizing the 1st-polymerized mixture containing unreactedmonomers completely by adding 0.03 g of said initiator (VA-044) to saidmixture for 3˜15 hours, vii) adding 64.557 g of sodium sulfate to thepolymer, viii) adjusting pH of the polymer less than 4.0, ix) sievingthe polymer using 60 mesh.

EXAMPLE 2 Preparation of the Polymer Dispersion

The polymer dispersion is prepared by following steps of: i) mixing83,004 g of 50% acrylamide aqueous solution, 172.279 g ofdimethylaminoethyl acrylate benzyl chloride quaternary monomer (80%),88.343 g of dimethylaminoethyl acrylate methyl chloride quaternarymonomer (80%), 12.5 g of dimethylaminoethyl acrylate methyl chloridequaternary polymer (20%), 3.125 g of dimethyldiallyl ammonium chloridepolymer (40%), 140.625 g of ammonium sulfate, 1.25 g of aromaticethoxylate, 62.5 g of cationic colloidal silica (20%) and 389.499 g ofwater in the 5-neck flask, ii) nitrogen purging in the reactor, andagitating said mixture to 200 rpm, iii) heating said mixture at 37° C.,iv) adding 0.05 g of azobis isonitrile initiator (VA-044) to saidmixture, v) 1st-polymerizing said mixture at 37° C for 3˜6 hours, vi)2nd-polymerizing the 1st-polymerized mixture containing unreactedmonomers completely by adding 0.05 g of said initiator (VA-044) to saidmixture for 3˜15 hours, vii) adding 46.875 g of ammonium chloride to thepolymer, viii) adjusting pH of the polymer less than 4.0, ix) sievingthe polymer using 60 mesh.

EXAMPLE 3 Preparation of the Polymer Dispersion

The polymer dispersion is prepared by following steps of: i) mixing349.833 g of 50% acrylamide aqueous solution, 31.355 g ofdimethylaminoethyl acrylate methyl chloride quaternary monomer (80%),8.0 g of dimethylaminoethyl acrylate methyl chloride quaternary polymer(20%), 2.0 g of dimethyldiallyl ammonium chloride polymer (40%), 150.591g of ammonium sulfate, 0.8 g of aromatic ethoxylate, 33.333 g offunctional colloidal silica (30%), 6.0 g of glycerin, 0.3 g ofethylenediaminetetraacetate (EDTA) and 352.379 g of water in the 5-neckflask, ii) nitrogen purging in the reactor, and agitating said mixtureto 200 rpm, iii) heating said mixture at 35° C., iv) adding 0.05 g ofazobis isonitrile initiator (VA044) to said mixture, v) 1st-polymerizingsaid mixture at 35° C. for 3˜6 hours, vi) 2nd-polymerizing the1st-polymerized mixture containing unreacted monomers completely byadding 0.05 g of said initiator (VA-044) to said mixture for 3˜15 hours,vii) adding 65.409 g of ammonium sulfate to the polymer, viii) adjustingpH of the polymer less than 4.0, ix) sieving the polymer using 60 mesh.

EXAMPLE 4 Test of Retention Effect

The printing and liner paper is prepared by using following pulps of: i)pulp containing 400 ppm of polymer dispersion of example 1, ii) pulpcontaining 400 ppm of polymer dispersion of example 2, iii) pulpcontaining 400 ppm of polymer dispersion of example 3, iv) pulp notcontaining retention aid (A), v) pulp containing commercially marketedcationic polymer among retention aid (B), vi) pulp containing cationicpolymer and bentonite in a ratio of 100: 1.0 (C), and vii) pulpcontaining cationic polymer and bentonite in a ratio of 100:3.0 (D). Theresult is shown in Table 1 and 2.

TABLE 1 OPR (%) FINE OPR (%) Quality of paper Example 1 92.0 80.0 goodExample 2 91.0 78.0 good Example 3 89.5 75.0 good A 68.2 13.1 bad B 75.545.8 bad C 83.3 65.0 bad D 87.3 73.5 bad OPR : one pass retention FINEOPR : one pass retention of ash Experimental condition: i) Paper :printing paper (70 g/m²) ii) Concentration of pulp : 0.315% iii) pH ofpulp : 6.80 iv) Instrument : Britt jar

TABLE 2 OPR (%) Quality of paper Example 1 91.8 good Example 2 90.6 goodExample 3 89.4 good A 72.2 bad B 80.8 bad C 83.2 bad D 85.5 bad OPR :one pass retention Experimental condition: i) Paper : liner paper (190g/m²) ii) Concentration of pulp : 0.510% iii) pH of pulp : 6.10 iv)Instrument : Britt jar

What is claimed is:
 1. A hydrophilic polymer dispersion containing acolloidal silica prepared by following steps of: i) mixing 1.0˜25 wt %of acrylamide, 0˜2 wt % of anionic monomer selected from the groupconsisting of acrylic acid, methacrylic acid and itaconic acid, 1.0˜30wt % of cationic monomer mixture of compound of formula I and formulaII, 0.5˜5 wt % of polymer selected from the group consisting ofhomopolymer of compound of formula I, homopolymer of compound of formulaII and copolymer of compound of formula I and formula II, 10˜30 wt % ofanionic salt, 0.5˜10 wt % of colloidal silica, 0.01˜1 wt % of nonionicsurfactant, 0.05˜2 wt % of dispersion stabilizer other than the nonionicsurfactant, and 40˜75 wt % of water; ii) 1^(st)-polymerizing the mixturewith the addition of 0.001˜0.1 wt % of polymerization initiator to saidmixture; iii) 2^(nd) polymerizing unreacted monomers in the1^(st)-polymerized mixture; and iv) adding and mixing 10˜30 wt % ofanionic salt to obtained polymers

 wherein R₁ is hydrogen atom or methyl; R₂ and R₃ are each independentlyalkyl group having 1 to 3 carbon atoms; A₁ is oxygen atom or NH; B₁ isalkylene group having 2 to 4 carbon atoms or hydropropylene; and X₁ isanionic counter ion

 wherein R₄ is hydrogen atom or methyl; R₅ and R₆ are each independentlyalkyl group having 1 to 2 carbon atoms; R₇ is hydrogen atom or alkylgroup having 1 to 2 carbon atoms; A is oxygen atom or NH; B₂ is alkylenegroup having 2 to 4 carbon atoms or hydropropylene; and X₂ is anioniccounter ion.
 2. The hydrophilic polymer dispersion according to claim 1,wherein said colloidal silica has 10˜30 nm of diameter.
 3. Thehydrophilic polymer dispersion according to claim 1, wherein saidanionic salt is selected from the group consisting of ammonium sulfate,ammonium chloride, sodium sulfate, magnesium sulfate, aluminium sulfate,ammonium hydrogenphosphate, sodium hydrogenphosphate, and potassiumhydrogenphosphate.
 4. The hydrophilic polymer dispersion according toclaim 1, wherein said dispersion stabilizer is selected from the groupconsisting of nonionic surfactant and glycerin.
 5. The hydrophilicpolymer dispersion according to claim 1, wherein the ratio of thecationic monomer of formula I and formula II is in the range of 10:0 to2:8.
 6. A process for preparing hydrophilic polymer dispersioncontaining a colloidal silica comprising the steps of: i) mixing 1.0˜25wt % of acrylamide, 0˜2 wt % of anionic monomer selected from the groupconsisting of acrylic acid, methacrylic acid and itaconic acid, 1.0˜30wt % of cationic monomer mixture of compound of formula I and formulaII, 0.5˜5 wt % of polymer selected from the group consisting ofhomopolymer of compound of formula I, homopolymer of compound of formulaII and copolymer of compound of formula I and formula II, 10˜30 wt % ofanionic salt, 0.5˜10 wt % of colloidal silica, 0.01˜1 wt % of nonionicsurfactant, 0.05˜2 wt % of dispersion stabilizer other than the nonionicsurfactant, and 40˜75 wt % of water; ii) 1^(st)-polymerizing the mixturewith the addition of 0.001˜0.1 wt % of polymerization initiator to saidmixture; iii) 2^(nd)-polymerizing unreacted monomers in the1^(st)-polymerized mixture; and iv) adding and mixing 10˜30 wt % ofanionic salt to obtained polymers

 wherein R₁ is hydrogen atom or methyl; R₂ and R₃ are each independentlyalkyl group having 1 to 3 carbon atoms; A₁ is oxygen atom or NH; B₁ isalkylene group having 2 to 4 carbon atoms or hydropropylene; and X₁ isanionic counter ion

 wherein R₄ is hydrogen atom or methyl; R₅ and R₆ are each independentlyalkyl group having 1 to 2 carbon atoms; R₇ is hydrogen atom or alkylgroup having 1 to 2 carbon atoms; A₂ is oxygen atom or NH; B₂ isalkylene group having 2 to 4 carbon atoms or hydropropylene; and X₂ isanionic counter ion.
 7. The process for preparing hydrophilic polymerdispersion containing a colloidal silica according to claim 6, whichcomprises the steps of: i) mixing 1.0˜25 wt % of acrylamide, 0˜2 wt % ofanionic monomer, 1.0˜30 wt % of cationic monomer mixture of compound offormula I and formula II, 0.5˜5 wt % of polymer selected from the groupconsisting of homopolymer of compound of formula I, homopolymer ofcompound of formula II and copolymer of compound of formula I andformula II, 10˜30 wt % of anionic salt, 0.5˜10 wt % of colloidal silica,0.01˜1 wt % of nonionic surfactant, 0.05˜2 wt % of dispersion stabilizerother than the nonionic surfactant, and 40˜75 wt % of water in thereactor; ii) nitrogen purging in the reactor, and agitating said mixturemore than 30 rpm; iii) heating said mixture at 20-50° C.; iv) adding0.001˜0.1 wt % of initiator to said mixture; v) 1^(st)-polymerizing saidmixture at 20˜50° C. for 3˜6 hours; vi) 2^(nd)-polymerizing unreactedmonomers in the 1^(st)-polymerized mixture by adding 0.001˜0.1 wt % ofinitiator to said mixture for 3˜15 hours; vii) adding the remaininganionic salt to the polymer; viii) adjusting pH of the polymer less than4.0; and ix) sieving the polymer using 40˜100 mesh.